Cyanate esters have been used as curable resins with desirable electrical and thermal properties. As such, they have found utility as matrix resins in a number of industrial applications. Such applications include printed circuit boards, antenna coatings, structural composites, encapsulating resins, matrix resin for abrasives, and adhesives. Chemistry and applications of cyanate esters are discussed in "The Chemistry and Technology of Cyanate Esters" by I. A. Hamerton .COPYRGT. 1994 Blackie Academic & Professional, an imprint of Chapman & Hall and references therein.
Cyanate ester resins, as described in U.S. Pat. No. 3,553,244, are produced by reacting a phenolic compound with cyanogen halide. Such cyanate esters, upon curing, are known to form hard thermoset matrices through cyclotrimerization of the cyanate ester groups. The cyclotrimerization produces aryloxytriazine rings which serve as the crosslink sites in the thermoset matrix. The cure of these resins is effected by heating, particularly in the presence of catalysts such as those described in U.S. Pat. Nos. 4,330,658, 4,330,669, 4,785,075, and 4,528,366. Curable compositions containing cyanate ester prepolymers are also known and are described in U.S. Pat. No. 4,740,584. Such prepolymers comprise cyanate ester-containing aryloxytriazine residues that can be further cured through the cyclotrimerization of the remaining cyanate ester moieties. Blends of cyanate ester prepolymers are described in U.S. Pat. Nos. 4,110,364 and 4,371,689. Blends of cyanate esters with thermoplastic polymers are disclosed in U.S. Pat. Nos. 4,157,360, 4,983,683, and 4,902,752.
Many of the aforementioned compositions, however, are not flame retardant and their use in applications, where flame retardancy is critical, is limited. Such applications include electrical applications such as printed circuit boards. Flame retardant cyanate ester blends are described in Japanese Patent No. 05339342 and U.S. Pat. No. 4,496,695, which describes blends of cyanate esters and brominated epoxies, or poly(phenylene ether) (PPE), cyanate esters and brominated epoxies. Epoxy resins however are known to have inferior electrical properties relative to cyanate esters, and the corresponding cyanate ester-epoxy blends do not have optimal electrical properties.
These issues have been addressed by preparing blends of brominated cyanate esters as disclosed in U.S. Pat. Nos. 4,097,455 and 4,782,178. Blends of cyanate esters with the bis(4-vinylbenzylether)s or brominated bisphenols are also described in U.S. Pat. Nos. 4,782,116, and 4,665,154. Blends of cyanate esters with brominated poly(phenylene ether)s, polycarbonates or pentabromobenzylacrylates are disclosed in Japanese Patent No. 08253582.
However, there is still a need for curable compositions comprising cyanate esters with optimal flame retardancy and electrical properties.